(1) Field of the Invention
This invention relates to a process for the preparation of zeolites having 5-membered rings of oxygen atoms. More particularly, it relates to a process in which zeolites having 5-membered rings of oxygen atoms and having a high purity and a high SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio can easily be prepared with a good reproducibility without using an organic mineralizer.
(2) Description of the Prior Art
As is seen from the fact that the term "zeolite" is derived from the Greek word "zein" (boiling stone), the zeolite is a crystalline aluminosilicate containing zeolitic water, which has an oxide molar composition represented by the following general formula: EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
wherein M stands for a metal cation, n stands for the valency of the metal cation M, x is a number of at least 2, and y is a number larger than 0.
The basic structure of the zeolite comprises SiO.sub.4 tetrahedrons having four oxygen atoms at apexes with the silicon atom being as the center and AlO.sub.4 tetrahedrons having four oxygen atoms at apexes with the aluminum atom being as the center, where these SiO.sub.4 tetrahedrons and AlO.sub.4 tetrahedrons are regularly and three-dimensionally connected to one another while owning oxygen atoms jointly. A three-dimensional network structure having pores differing in the size and shape according to the manner of connection of the tetrahedrons can be provided. Negative charges of the AlO.sub.4 tetrahedrons are electrically neutralized by coupling with cations of an alkali metal or alkaline earth metal. The thus-formed pores have a size of 2 to 10 angstroms or more, and the pore size can be changed by exchanging the metal cations connected to the AlO.sub.4 tetrahedrons with other metal cations having a different size.
The zeolite is widely used as a dehydrating agent for a gas or liquid or as a molecular sieve for adsorbing and separating specific molecules by utilizing the above-mentioned pores. Furthermore, the zeolite is used on an industrial scale as a catalyst by utilizing properties as the solid acid.
Various zeolites differing in the crystal structure and chemical composition are known, and the adsorbing properties, catalytic properties and ion-exchanging properties differ according to the differences of the crystal structure and chemical composition. Different names are given to these zeolites. Among these zeolites, mordenite, offretite, erionite, clinoptilolite and chabazite are naturally present, and they can also be obtained by the hydrothermal synthesis. Zeolites A, X and Y are not naturally present, but they are obtained only by the hydrothermal synthesis.
The zeolites prepared by the process of the present invention are classified into two types, one of which is similar to a ZSM-5 type and the other of which is a ferrierite type.
Zeolite ZSM-5 is formed by the hydrothermal synthesis using as an indispensable starting material an organic mineralizer such as tetrapropyl ammonium ion and is a crystalline aluminosilicate of the orthorhombic system having lattice constants of a=20.1 .ANG., b=19.9 .ANG. and c=13.4 .ANG. and having lattice spacings (d-values), shown in Table 1, of the X-ray powder diffraction pattern. In the crystal of this zeolite, tetrapropyl ammonium ion is present as a part of the cations constituting the crystal.
TABLE 1 ______________________________________ Lattice Spacing (d-value) Relative Intensity ______________________________________ 11.1 .+-. 0.3 Strong 10.0 .+-. 0.3 Strong 7.4 .+-. 0.2 Weak 7.1 .+-. 0.2 Weak 6.3 .+-. 0.2 Weak 6.04 .+-. 0.2 Weak 5.56 .+-. 0.1 Weak 5.01 .+-. 0.1 Weak 4.60 .+-. 0.08 Weak 4.25 .+-. 0.08 Weak 3.85 .+-. 0.07 Very strong 3.71 .+-. 0.05 Strong 3.04 .+-. 0.03 Weak 2.99 .+-. 0.02 Weak 2.94 .+-. 0.02 Weak ______________________________________
Various processes have been proposed and disclosed as the improved process for the synthesis of ZSM-5 or a zeolite similar to ZSM-5. For example, a process using an organic sulfur compound as the organic mineralizer agent is disclosed in Japanese Unexamined Patent Publication No. 54-137,500, a process using a cyclic ether is disclosed in Japanese Unexamined Patent Publication No. 56-140,023, a process using an alcohol amine is disclosed in Japanese Unexamined Patent Publication No. 56-17,920, and a process using an amino acid is disclosed in Japanese Unexamined Patent Publication No. 56-7,817. In these processes, use of an organic mineralizer is indispensable. Recently, a trial has been made to synthesize a zeolite similar to ZSM-5 without using an organic mineralizer. For example, Japanese Unexamined Patent Publication No. 57-7,819 teaches a process in which the synthesis is carried out in the presence of a seed crystal synthesized in the presence of tetrapropyl ammonium bromide.
Furthermore, Japanese Examined Patent Publication No. 56-49,851 discloses a process in which a zeolite similar to ZSM-5 is synthesized in the absence of an organic mineralizer by using a silica sol as the silica source.
From the results of the research made by us, it was found that it is very difficult to synthesize a zeolite similar to ZSM-5 without using an organic mineralizer and this zeolite is not always synthesized at a high efficiency even when the synthesis is carried out faithfully according to the known processes. It also was found that the above zeolite cannot be synthesized unless reaction conditions such as the starting silica component, the mixing order of the starting materials, the stirring condition and the crystallization temperature are selected in a complicated manner. For example, it was found that in the case where a silica sol customarily used in this field is used as the silica source, if stirring is effected during the crystallization, a zeolite similar to ZSM-5 cannot be obtained even when the composition of the starting materials and the crystallization temperature are controlled in various manners. Furthermore, in most of experiments where stirring was effected only at the step of preparing the starting materials and heating was effected without stirring at the other steps, the product was amorphous or was composed mainly of mordenite. Exceptionally, formation of a zeolite similar to ZSM- 5 was observed, but in this case, impurities such as mordenite were inevitably formed simultaneously with the zeolite.
A zeolite similar to ZSM-5 was obtained only when an aqueous solution of sodium silicate was used as the silica source, fine ZSM was added as a seed crystal and crystallization was effected under heating with stirring. However, mordenite and kenyaite were present as impurities in the product. Even if various experiments were performed without addition of a seed crystal by using an aqueous solution of sodium silicate as the silica source, the intended object could not be attained.
A ferrierite type zeolite naturally occurs, and its typical composition is represented as follows: EQU (Na.sub.2,Mg)O.Al.sub.2 O.sub.3.11.1SiO.sub.2.6.5H.sub.2 O
The crystal structure of this zeolite comprises 5-membered oxygen ring units constituting the skeleton and is characterized in that it has fine pores of 10-membered rings of oxygen atoms having a size of 4.3.times.5.5 .ANG. and fine pores of 8-membered rings of oxygen atoms having a size of 3.4.times.4.8 .ANG..
Various processes as described below have been proposed as the process for synthesizing ferrierite type zeolites. However, an industrially satisfactory process has not been developed.
(1) In the process proposed by Mr. C. L. Kibby et al (Journal of Catalysis, Vol. 35, pages 256-272, 1974), a high temperature exceeding about 300.degree. C. is necessary for crystallization, and therefore, a high-temperature high-pressure type reaction for crystallization, and therefore, a high-temperature high-pressure type reaction vessel should inevitably be used.
(2) In the process disclosed in Japanese Unexamined Patent Publication No. 51-106,700, the synthesis can be carried out at a relatively low temperature, but a starting silica-alumina material having a specific composition should be prepared and the presence of a potassium ion in the reaction system is indispensable. Moreover, addition of a mineralizer such as a sodium salt and/or potassium salt of an organic or inorganic polybasic acid is indispensable. Thus, in this process, selection of the starting material and setting of the reaction conditions are complicated, and the process is not advantageous from an industrial viewpoint.
(3) In the process disclosed in Japanese Unexamined Patent Publications No. 50-127,898 and No. 55-85,415, use of N-methylpyridine hydroxide and piperidine and/or an alkyl-substituted piperidine as an organic mineralizer is indispensable. These organic amines are expensive, and since these organic amines are included in the formed zeolite, when the ferrierite type zeolite obtained according to this process is used as an adsorbent or catalyst, it is necessary to calcine the synthesized zeolite at a high temperature of at least 500.degree. C. in the presence of oxygen to remove the amines. Namely, when the zeolite is used for these specific purposes, the zeolite should be subjected to a preliminary treatment as described above.
(4) The process disclosed in Japanese Unexamined Patent Publication No. 53-144,500 is a process for synthesizing a ferrierite type zeolite called "ZSM-35", in which butanediamine or an organic base-containing cation derived therefrom is used. As is the above-mentioned process (3), the zeolite obtained by the synthesis should be subjected to a preliminary treatment before it is actually used in various fields.
It is known that in zeolites comprising 5-membered rings of oxygen atoms as the zeolite skeletone-constituting units, such as ferrierite, mordenite and ZSM-5, the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio can be increased to a relatively high level. However, for the synthesis of these zeolites, it is ordinarily indispensable that an organic nitrogen-containing compound or other organic compound should be present in the reaction system, as pointed out hereinbefore. Furthermore, in these known processes, in order to enhance the activity of the reaction mixture, expensive aqueous colloidal silica is ordinarily used as the silica source.